1. Field of the Invention
The present invention relates to unsaturated elastomeric ABC block copolymers based on butadiene, isoprene and styrene, their production, and their use for the manufacture of tire treads.
2. Description of the Background
It is known that rubbers which are employed in the manufacture of tires are subject to the following demands:
The cold flow of the rubber should be as low as possible;
The rubbers should be readily processable in the subsequent mixing processes;
The rubbers should be free-flowing in molding processes;
The rubbers should be readily vulcanizable.
Further, demands in tire manufacture arise from their field of use. As is known, improvements in the properties of tire treads are in demand.
a) Tires surfaces should be highly elastic, even at low temperatures. PA1 b) Tires surfaces should have a good grip in wet condition. PA1 c) Tires surfaces should have high abrasion resistance and accordingly a long service life. PA1 d) Tires surfaces should generate as little heat as possible on dynamic load. Their rolling resistance should be as low as possible so that the fuel consumption of the vehicle is as low as possible. PA1 1. The block copolymers do not satisfy the above-mentioned demands of a tire material; PA1 2. Problems occur in the compatibility of the two blocks; PA1 3. The damping curve has only a narrow damping maximum; PA1 4. Large amounts of comparatively expensive isoprene are necessary. PA1 i) 40 to 75% of block A which contains styrene and butadiene units or isoprene and butadiene units with a content of distributed vinyl or isopropenyl groups of less than 15%, PA1 iia) up to 25% of a block B which contains styrene and butadiene units or isoprene and butadiene units with a vinyl or isopropenyl content of more than 70%, or PA1 iib) up to 25% of a block B' which contains styrene, isoprene and optionally butadiene units with a content of homogeneously distributed isopropenyl or vinyl groups of less than 15%, and PA1 iii) 20 to 55% of a block C which contains styrene and isoprene units and optionally butadiene units with an isopropenyl or vinyl content of more than 70%.
It is known that, when tested in the torsion pendulum test, rubbers exhibit a temperature dependency of the logarithmic decrement of the mechanical damping which corresponds, when presented in the form of a graph, to a curve which is characteristic of the particular rubber. The demands desired on the tire treads arise preferentially when the damping curve has the broadest possible vibration-damping range (cf. K. H. Nordsiek, Kautschuk und Gummi, Kunststoffe 38, 178 (1985) and 39, 599 (1986).
It is also known that the properties of the tire treads, some of which are partly mutually ccnflicting, are determined to a very considerable extent by the type and composition of the rubbers employed for this purpose. Homopolymers based on the monomeric raw materials usually employed, such as butadiene, isoprene and styrene, still fail to fulfill these conditions satisfactorily (cf. EP-OS 0,054,204 and JP-OS 82/87,406l ).
In practice, blends of rubber varieties have the disadvantage that the range of properties mentioned is not achieved and the tire quality desired is not reproduced reliably There is, therefore, a demand for rubbers which, as far as possible, approach the property expectations given above. This aim should in principle be achievable by means of rubbers which comprise polymers containing different blocks.
For the purposes of this invention, blocks of a polymer should be regarded not only as chain segments comprising different monomer components, but also segments which differ, as a consequence of the external process parameters, in the type of linking of the monomer components or in the proportion in which they are incorporated into a chain segment.
Although the butadiene-isoprene copolymer described in EP-OS 0,054,204 has different contents of isoprene in its initial and terminal parts, which is attributable to the low polymerization tendency of isoprene compared with butadiene, it cannot be regarded as a block copolymer in the sense described above. Even if the proportion of styrene is altered during the copolymerization of dienes and styrene (cf. DE-OS 3,108,583), it is not a block copolymer which is obtained, but rather a polymer having gradual transitions. The desired improvement in tire properties is also still inadequate here. DE-OS 3,108,583 describes single-phase rubber systems having a damping maximum caused by a glass transition temperature in a very narrow temperature range. An improvement, however, is not achieved until a copolymer comprising two different blocks A and B is prepared which differ in structure and/or composition. Thus, for example, DE-OS 3,151,139 describes a random styrene-butadiene block copolymer. The blocks differ in their butadiene content and in the content of vinyl bonds. They are mixed with one another in a manner such that they are compatible and such that the two separate damping peaks are replaced by only one.
In DE-OS 3,530,438, rubber compounds are claimed which contain at least 20% of a styrene-butadiene block copolymer. The blocks differ in their styrene contents, in their contents of vinyl bonds and, as a consequence, in the glass transition temperatures. In this case too, the damping curve has only a narrow temperature range of maximum damping.
Japanese Laid Open Application 83/122,907 describes branched rubbers which can be obtained by reacting, for example, a metallic tetrahalogen compound such as SnCl.sub.4 with block copolymers comprising a polyisoprene block and a polybutadiene block. Each of the two blocks thus is a homopolymer. The star-shaped rubber obtained after reaction with the metallic coupling agent forms a single-phase rubber system having a glass transition point.
British Patent 2,090,840 describes block copolymers which are obtained by polymerization of dienes or copolymerization of diene mixtures, wherein the blocks differ by 20 to 50 mol-% in the content of 1,2 and/or 3,4 structural units. Block copolymers of this type are produced in the presence of various amounts of cocatalyst or at various temperatures.
EP-OS 0,173,791 describes tread materials whose rubber component may comprise 30 to 100% of block copolymers based on butadiene, isoprene and, if appropriate, styrene and/or piperylene. The block copolymers are produced in the presence of cocatalysts while increasing the temperature and may, for example, have an ABC structure. The polymers always contain an end block based on butadiene which is produced at increasing temperature and consequently has a comparatively high proportion of 1,2 structural units and an inhomogeneous distribution of vinyl groups. These block copolymers again fail to produce damping curves which exhibit a sufficiently broad plateau which is necessary to produce all tire properties in an ideal manner (see comparative experiment A). For this reason, it has already been proposed in that application to blend the block copolymers obtained with other rubber components (see claim 1 and Example 2).
All the block copolymers described above are unsatisfactory for at least one of the following reasons:
A need continues to exist for a rubber material of improved properties as the rubber for the tire treads.